The present invention relates to a coated steel sheet used mainly as a steel sheet for car bodies and a method for making the same, and particularly, relates to a coated steel sheet that has excellent perforative corrosion resistance with no-painting, as well as after electrodeposition painting, chemical conversion treatability and press formability.
A galvanized steel sheet is broadly used to prevent the strength of a car body from deteriorating after long-term use under a corrosive environment. In Japan, as zinc alloy plating, a zinc-nickel alloy plated steel sheet and a zinc-iron alloy plated steel sheet are mainly used.
The zinc-based alloy plating can provide high corrosion resistance to a steel sheet by alloying Ni or Fe and zinc, but there are some problems with alloy plating.
For instance, although a zinc-nickel alloy plated steel sheet is made by electroplating, Ni is expensive and the cost increases thereby. A problem is also found in that Ni content has to be normally controlled in an extremely narrow range (for instance, 12xc2x11 mass %) and the making is difficult.
On the other hand, a zinc-iron alloy plated steel sheet may be made by either electroplating or hot dipping.
However, in producing a zinc-iron alloy plated steel sheet by electroplating, as for a zinc-nickel alloy plated steel sheet, it is difficult to control a percentage content of iron in a galvanized layer within an extremely narrow range, which is so-called alloy control. Furthermore, Fe2+ ions in plating solution are likely to be oxidized, so that plating becomes unstable and making will be difficult. Accordingly, there is a problem in that the costs will be high.
In general, a zinc-iron alloy plated steel sheet is often made by hot dipping. In making a zinc-iron alloy plated steel sheet by hot dipping, a steel sheet is kept at high temperature after molten zinc is adhered to the sheet surface, thus alloying the steel sheet and zinc. However, in this method, quality fluctuates significantly, depending on Al concentration in a galvanizing bath and the temperature and time of an alloying process. A highly advanced technology is necessary to make a uniform alloy plated layer. As a result, as expected, the costs will be high.
As indicated above, any zinc-based alloy plating has problems in that the producing is difficult and the costs will be high.
On the other hand, a galvanized steel sheet in which only zinc is plated, may be made by either electroplating or hot dipping at a low cost. However, the sheet has been rarely used for a car body. This is because corrosion resistance is insufficient only with zinc plating. Especially, when a galvanized steel sheet is exposed to a corrosive environment over a long period, the sheet is likely to have perforative corrosion and has a problem in guaranteeing the strength of a car body. Additionally, a large amount of zinc is likely to accumulate on electrodes during spot welding. The endurance of electrodes is shortened, and press formability is poor.
Normally, in producing a car body, a steel sheet or a galvanized steel sheet is welded after press forming. Furthermore, after sequentially performing a chemical conversion treatment, electrodeposition painting and spray coating, the sheet is used for a car body. It is also generally known that a lower part of a door in a car body is most likely to have perforative corrosion. This is because the lower part is folded, and water that entered through window gaps and so forth is likely to accumulate therein, so that the lower part tends to corrode faster than other parts of a car body.
Among the treatments after the press forming of a car body, the chemical conversion treatment and the electrodeposition painting may be performed even at an inner side of a door, but paint cannot be applied thereto in the following spray painting. Thus, since anti-corrosion effects cannot be expected from spray painting, perforative corrosion resistance after electrodeposition painting becomes important. Additionally, at a folded part (hem structure) at a lower part of a door which is the most severely corroded section, chemical conversion treatment solution can be spread, but electrodeposition painting cannot be performed, and the part is directly exposed to a corrosive environment. Accordingly, perforative corrosion resistance becomes important in both cases with no electrodeposition painting (no-painting) and with electrodeposition painting only (after electrodeposition painting).
In this background, an art in which a Mg-containing coating is formed on zinc plating, is disclosed as a method to improve corrosion resistance of a galvanized steel sheet. For instance, Japanese Unexamined Patent Application Publication No. 1-312081 discloses a coated metal from which a phosphate coating containing Mg at 0.1 mass % or more is formed on an electrogalvanized layer.
The coated metal in the above-noted publication from which the phosphate coating containing only Mg is formed, is effective against rust in a salt spray test. However, the metal has insufficient perforative corrosion resistance in a composite cycle corrosion test, which is often the reflection of actual corrosion of a car body.
Moreover, Japanese Unexamined Patent Application Publication No. 3-107469 discloses a material from which a phosphate coating containing Mg at 1 to 7% is formed on an electrogalvanized layer. However, even in this case, although the material prevents rust in a salt spray test, perforative corrosion resistance in a composite cycle corrosion test is insignificant since only Mg is contained in the phosphate coating.
Furthermore, Japanese Unexamined Patent Application Publication No. 7-138764 discloses a zinc-containing metal plating steel sheet which is formed with zinc phosphate composite coating containing zinc and phosphorus at the weight ratio (zinc/phosphorus) of 2.504:1 to 3.166:1, and 0.06 to 9.0 weight % of at least one metal selected from iron, cobalt, nickel, calcium, magnesium and manganese, on a surface of a zinc-containing metal plated layer. However, although this plating steel sheet has excellent high-speed press formability, its corrosion resistance was not considered, and its perforative corrosion resistance is insignificant.
Additionally, Japanese Examined Patent Application Publication No. 55-51437 discloses a method of treating a galvanized steel sheet with aqueous solution containing magnesium biphosphate and condensed phosphate or boron compound, and treating the sheet with heat at 150 to 500xc2x0 C. In this method, corrosion resistance in a salt spray test improves. However, since paint adhesion under a corrosive humid environment is poor after electrodeposition painting, corrosion resistance is low and perforative corrosion resistance is insignificant.
Japanese Unexamined Patent Application Publication No. 4-24193 discloses that magnesium oxide or magnesium hydrated oxide is deposited on a galvanized steel sheet at 10 to 5000 mg/m2. Even in this method, like the method mentioned above, corrosion resistance in a salt spray test improves. However, since paint adhesion under a corrosive humid environment is poor after electrodeposition painting, corrosion resistance after electrodeposition painting is low and perforative corrosion resistance is insignificant.
Japanese Unexamined Patent Application Publication No. 58-130282 discloses a method of contacting aqueous solution containing Mg at 10 to 10000 ppm, to a galvanized steel sheet after a chemical conversion treatment. Since the chemical conversion treatment is carried out over zinc plating in this method, paint adhesion improves. However, perforative corrosion resistance after electrodeposition painting and with no-painting is insignificant as ordinary Mg salts (chloride, sulfate, oxide, and so forth) are used.
Japanese Unexamined Patent Application Publication No. 59-130573 discloses a method of contacting aqueous solution of pH 2 or higher containing iron ions and magnesium ions at the total of 5 to 9000 ppm, to a galvanized steel sheet after a phosphate treatment. Since the phosphate treatment is carried out over zinc plating in this method, paint adhesion improves. However, perforative corrosion resistance after electrodeposition painting and with no-painting is insignificant since iron ions are contained in the treatment solution.
Japanese Unexamined Patent Application Publication No. 57-177378 discloses a pre-coating treatment in which aqueous solution containing an oxidation inhibitor such as phosphate or a precipitation inhibitor such as magnesium salt is adhered to a steel sheet after a phosphate coating is formed thereto, and then dried. A main component of the phosphate coating is iron phosphate, zinc phosphate, zinc-iron phosphate, calcium phosphate, and so forth. Additionally, the aqueous solution adhered thereafter is simple aqueous solution of phosphate, magnesium salt, so that perforative corrosion resistance after electrodeposition painting and with no-painting is insufficient.
Japanese examined Patent Application Publication No. 59-29673 discloses a method of coating aqueous solution which contains myo-inosital phosphate, Mg salt and so forth, and water soluble resin, to a zinc or zinc alloy plated steel sheet. Application with no-painting, or improvement of corrosion resistance in a storage period before painting is an object of this method, substituting for a zinc phosphate chemical conversion coating as a conventional painting substrate. On the other hand, in the application in which a chemical conversion treatment is carried out before painting, it is an object to easily have a coating fall off during a degreasing process and to form zinc phosphate crystals uniformly. According to the invention, a coating falls off in a chemical conversion treatment of automobile producing steps, so that corrosion resistance at parts where electrodeposition painting is not performed in the electrodeposition painting process, does not improve at all an actual perforative corrosion resistance of a car body is insignificant. Additionally, press formability as a problem of galvanization hardly improves. The corrosion resistance after painting also does not exceed that of conventional zinc phosphate coating.
The object of this invention is to provide a coated steel sheet from which a coating does not fall off, as described later, even in a chemical conversion treatment of an automobile producing line, and a sheet having excellent perforative corrosion resistance with no-painting and as well as after electrodeposition painting, chemical conversion treatability and press formability, and which is useful as a rust preventive steel sheet for a car body, and the method for making the same.
The inventors have devoted themselves to discovering the methods to solve the problems in conventional arts. Accordingly, the inventors have invented a coated steel sheet which has a zinc phosphate based coating containing Mg on the surface of a galvanized steel sheet, and moreover, has an orthophosphoric acid ester-containing coating on the surface of the zinc phosphate coating.
It is preferable that the zinc phosphate coating further contains Ni and Mn since the perforative corrosion resistance of the coated steel sheet after electrodeposition painting further improves. In this case, it is further preferable that the zinc phosphate coating contains Mg at 0.5 to 10.0 mass %, Ni at 0.1 to 2.0 mass % and Mn at 0.5 to 8.0 mass %, and that the contents of Mn and Ni satisfy the following Formula (1). Accordingly, perforative corrosion resistance after electrodeposition painting improves significantly.
[Ni]xc3x977.6xe2x88x9210.9xe2x89xa6[Mn]xe2x89xa6[Ni]xc3x9711.4xe2x80x83xe2x80x83(1)
wherein [Mn] is mass % of Mn, and [Ni] is mass % of Ni.
Additionally, in the above-mentioned conditions, the contents of Mg, Ni and Mn in the zinc phosphate coating, in particular, are further limited to a specific narrow range. Specifically, the above zinc phosphate coating contains Mg at 2.0 to 7.0 mass %, Ni at 0.1 to 1.4 mass % and Mn at 0.5 to 5.0 mass %, and the contents of Mn and Ni satisfy the Formula (1) mentioned above. Accordingly, both perforative corrosion resistance and press formability improve, which is more preferable. It is further preferable, in case of the coated steel sheet, that zinc phosphate in the zinc phosphate coating is granular crystals of less than 2.5 xcexcm of the longer side since press formability particularly improves further.
It is further preferable that the orthophosphoric acid ester-containing coating additionally contains Mg since perforative corrosion resistance of any of the coated steel sheets mentioned above improves further.
Moreover, the present application also provides a method for producing a coated steel sheet in which a galvanized steel sheet is treated with zinc phosphate treatment solution containing Mg, and is subsequently coated with aqueous solution containing orthophosphoric acid ester and is then dried.
It is preferable that the aqueous solution containing orthophosphoric acid ester further contains Mg in the method. In this case, it is further preferable that the aqueous solution containing orthophosphoric acid ester contains Mg at 2 to 30 g/l and orthophosphoric acid ester at 5 to 500 g/l.
Moreover, the orthophosphoric acid ester is preferably at least one kind selected from the group consisting of triaryl phosphate, hexose monophosphate, adenylic acid, adenosine diphosphate, adenosine triphosphate, phytic acid, inosinic acid, inosine diphosphate, and inosine triphosphate in each method mentioned above.
Furthermore, Mg that is contained in the zinc phosphate treatment solution or the orthophosphoric acid ester-containing aqueous solution, is preferably supplied from at least one type selected from the group consisting of magnesium hydroxide, magnesium oxide, magnesium nitrate, magnesium silicate, magnesium borate, magnesium hydrogenphosphate, and trimagnesium phosphate in any method mentioned above.